Let's say I have a lvl 100 Garchomp and five lvl 1 pokémon in my party, Exp. Share turned on. Can I use my Garchomp to KO pokémon and still get the right amount of EV's for my other pokémon via Exp. Share? And should the Garchomp or the lvl 1 pokémon hold the Power Item for the additional 8 EV's?
Thanks!
In Generation VI, it is now a Key Item, and experience and effort values are given to all of the Pokémon in the party individually. Pokémon that do not participate in battle will only gain 50% of the possible experience.
Yes. If you have the Exp. Share on and defeat a Pokemon, all of the Pokemon in the party will gain the EVs for defeating that Pokemon.
As for power items:
In Generation VI, the Exp. Share was made into a Key Item, meaning that Pokémon who receive experience from the Exp. Share can also take advantage of any held EV-enhancing items.
By the way this is worded, it makes me think that you'll have to give each individual Pokemon that you are EV training the power item in order for them to get the eight additional EVs.
Sources: This and this.